1. Field of the Invention
This invention relates to mechanisms for adjusting the pitch of propellers. More particularly, the invention relates to mechanisms having the capability of changing the pitch of blades in wind driven turbines, aircraft propellers and the like while the blade hub is rotating.
2. Description of Background Art
Rotating propellers are used extensively to move air or water, or to extract kinetic energy from moving water or air. Examples of propellers in the first category include aircraft and boat propellers. The second category includes water turbines used to generate hydroelectric power, and windmills coupled to electric generators. These are used to take advantage of moving water or wind as a source of energy.
In many instances, it is desirable to provide a variable pitch capability to the blades of a propeller. For example, a boat propeller used to propel a heavy boat at low speeds works more efficiently if the blades are twisted so that the planes of the blades make large angles with the rotational axis of the propeller. For high speed operation, boat propellers having a small pitch, characterized by small angles of attack of the propeller blades relative to their rotation circle, are more efficient.
The requirement for varying the pitch of propeller blades to optimize the performance of the propeller for varying speed and load conditions has prompted development of a number of designs for pitch control mechanisms. For example, Hepperle, in U.S. Pat. No. 2,309,899, issued Feb. 2, 1943 discloses a pitch control mechanism for aircraft propellers made of flexible wooden laminations in which an axially disposed helicoid gear is moveable longitudinally within an internally threaded propeller hub extension. Moving the helicoid gear causes the hub extension to be rotated with respect to a drive rod on which the helicoid gear is mounted, and also with respect to a hollow propeller shaft which coaxially encloses the drive rod. The mechanism includes a split collar having an annular flange which fits into an annular recess at the rear of the propeller hub. The halves of the split collar are held together by a lock ring. Flexible rods fixed in the short front lamina of a laminated propeller blade extend longitudinally backward through bores in intermediate length lamina, through bores in shorter rear lamina, and into adjacent bores in the split ring.
When the propeller hub is caused to rotate with respect to the hollow propeller shaft by axial movement of the drive rod, the flexible rods move out of parallel alignment with the shaft, causing the front ends of all of the flexible rods as well as the hub structures interconnected by the rods to move. The movement is progressively larger in the direction from substantially stationary rear lamina forwards to larger movement of front, short lamina, thus varying the pitch of the propeller blades.
O'Connor, in U.S. Pat. No. 2,953,208, issued Sept. 20, 1960 discloses a variable pitch marine propeller. Each blade of the propeller has a pivot shaft which extends outward from the base portion of the blade, near the leading edge of the blade. The pivot shafts are fastened to a retaining ring which fits over the hub of the propeller. The method of fastening secures the pivot shafts against axial movement, but permits rotation of each pivot shaft about its longitudinal axis. Circular transverse cross-section guide detents extending outward from the base of each blade's trailing edge ride within a helical groove machined into the outer surface of a guide control ring fitted coaxially over the outward end of the hub. Rotating the guide control ring with respect to the hub and the retainer ring causes movement of the detents in the helical groove, swinging the blades about the pivot shafts, causing the pitch of the propeller to change.
The examples of prior art pitch control mechanism cited above may be useful for their intended purposes. However, they are not well suited to use in wind turbine applications, for the following reasons. The Hepperle pitch control mechanism is heavy, cannot accommodate a large number of blades and requires a flexible external covering over each blade to maintain an airfoil. The O'Connor mechanism has a limited number of blades, and pitch can only be adjusted by hand when the propeller is stopped. Pitch control mechanisms for wind turbine applications require somewhat different performance characteristics. These characteristics will now be described.
To operate efficiently at low wind velocities, a wind turbine ideally has a larg- number of blades extending radially outward from a central hub, with the planes of the blades aligned nearly perpendicular to the wind vector. Wind turbines of this type, having a large number of blades and a small pitch, are typified by the farm windmills which were used extensively in this country during the first half of the twentieth century, primarily to power water pumps. Most old-style farm wind turbines did not have any provision for changing their pitch.
Presently used wind turbines usually have a small number of blades, typically two to four. To a large extent, present wind turbines are limited in the number of blades mounted on the turbine hub by the excessive weight of existing pitch control mechanisms. It is disadvantageous to decrease the number of propeller blades, since turbines having a larger number of blades can transmit a larger torque to the turbine shaft. Therefore, turbines with more blades are more efficient in converting wind energy into turbine shaft power. In most wind turbine applications, the turbine shaft is coupled to the input shaft of an electrical generator. Thus, more electrical power can be produced from a given amount of available wind power by turbines having a larger number of blades.
When a wind-driven turbine is exposed to high wind velocities, it is necessary to decrease the pitch of the propeller blades, to prevent ihe propeller shaft from rotating at excessive speeds. Reduction in pitch is accomplished by twisting the blades around their own longitudinal axes to position the plane of the blades more nearly parallel to the propeller axis, and to the wind vector. In extremely high winds, it is sometimes desirable to bring the pitch of the propeller blades to a position referred to as full feather. In this position, the flat surfaces of the propeller blades are made parallel to the propeller axis and to wind vector, so that little or no rotation of the propeller is caused by the wind.
The present invention was conceived of in response to a perceived need for a pitch control mechanism which is lighter, more durable, more easily constructed and maintained than prior disclosed pitch control mechanisms.